The present invention relates to managing computer data, and more specifically to cleaning cascaded volumes.
It is common for a storage system to have a disk backed up in the case of loss due to hardware malfunction and/or natural disaster. A backup disk may be created by various techniques. For example, a backup disk may be made using snapshot or cloning techniques.
A snapshot is a point-in-time copy that is dependent on the primary disk. A snapshot may, for example, be accomplished using a copy-on-write procedure, in which currently existing data in a region on the primary disk is written to the backup disk when a write is being made to the region on the primary disk. Thus, the backup disk will contain data that has been overwritten on the primary disk, as opposed to a complete copy of the primary disk. This type of backup copy typically results in a thinly provisioned volume, which reduces storage. A series of snapshot copies may be cascaded together to represent the primary disk at various times. However, the snapshot copies typically remain dependent on the primary disk to reassemble a complete copy of the primary disk.
A clone is a point-in-time copy that is independent of the primary disk. A clone may, for instance, be created by executing a background copy procedure in which a disk's regions are sequentially copied to the backup disk and executing a copy-on-write procedure to immediately copy any primary disk regions that are about to be overwritten due to a write and have not yet been processed by the background copy procedure. A clone is typically used when a copy is needed and input/output (IO) to the copy must not impact IO to the primary volume in any way. A clone may also be used when the copy is not to be affected by availability to the source. A clone may also be used in a cascade.